The magnetic recording industry has constantly and dramatically increased the performance and capacity of hard disk drives to meet the insatiable demand of the computer industry for more and better storage. Applications such as multimedia, real-time audio and video, graphical user interfaces and increasing program sizes are driving this increase. Hard disk areal density storage capacity has been increasing at an average yearly growth rate of at least 25 percent. Sustaining this growth in capacity has required progressive advances in many technologies used to provide a hard disk drive.
Historically, the read-write head technology was based on the inductive voltage produced when a permanently magnetized area on a rotating disk moved past a head employing a wire-wrapped magnetic core. Increasing areal density requirements drove a steady progression of inductive recording head advances, which culminated in advanced thin-film inductive read-write heads. Further advancements in this technology, and the ability to cost-effectively produce these heads has reached a point of diminishing return. Additionally, another critical limitation of the inductive head is that it must alternatively perform the conflicting tasks of writing data onto the disk and reading this previously-written data.
This limitation may be overcome by separating the write and read functions into two physically distinct heads. This allows using an inductive head that is optimized for writing data and a magnetoresistive head structure that is optimized for reading data. The magnetoresistive read head consists of a read element that is sandwiched between two highly-permeable magnetic shields. The shields assist in focusing the magnetic energy from the disk and rejecting stray fields. The magnetoresistive read element is made from a ferromagnetic alloy whose resistance changes as a function of an applied magnetic field. In the hard disk drive, this magnetic field is derived from the magnetized regions placed on the rotating disk by the write head and is used to modulate the resistivity of the magnetoresistive read element.
The magnetoresistive read element is biased using constant current sources that provide a constant bias current through the read element. As the resistivity of the read element changes due to the influence of the applied magnetic field, the voltage across the read element changes in direct proportion since the current through it is constant. This read element voltage is further conditioned by a high input impedance circuit to provide a read signal from the hard disk drive. Several problems arise due to this arrangement. The magnetoresistive read element is sensitive to voltage overstress and may be damaged if the voltage across it becomes too high. The bias current from the constant current sources is dynamically altered in an effort to prevent this voltage overstress. However, time delays associated with the high impedance circuits may be too long to avoid damage to the read head. Additionally, various device and parasitic capacitances combined with the high impedances also limit frequency bandwidth and signal responses.
What is needed in the art is a more effective way to bias a magnetoresistive head that reduces the present voltage overstress and signal response limitations.